It has been proposed that the high incidence of obesity in the Pima Indians may be due to genetic selection for a "thrifty gene", a gene that would allow the Pimas to store energy very efficiently; these efficient individuals would presumably be more capable of surviving famine than lean, less-efficient individuals. In order to help determine whether or not such a genetic difference exists, thermogenic respones to various stimuli, including food, norepinephrine, and exercise, are being investigated in obese Pima volunteers. These thermogenic responses are being measured during periods of weight maintenance, of overnutrition, and of undernutrition. Thyroid hormone metabolism and resting metabolic rate is also being studied, since previous studies with Caucasian volunteers have shown that these parameters are affected by dietary manipulation. The results from this research are being compared to those obtained from lean and obese Caucasians involved in studies in Vermont.